In the market of power-assisted wheel set, power-assisted vehicles have wheels. There is a hub in the center of each wheel and the hub is fastened to the wheel by screws. A power system of a power-assisted vehicle requires high reduction ratio and has certain size limit. Besides, when designing a power-assisted wheel, space for anchoring, electrical connection, etc. is needed in the central area of the wheel. If a common planetary gear system is chosen as a transmission unit, there will not be enough space in the central area of the wheel because of the tight arrangement of gears, or the reduction ratio will not be ideal in order to have more space. If a planetary gear system with small difference in numbers of teeth is chosen, the size of the transmission unit will be within the limit; however, the mechanism may be locked while a reverse driving force is applied.